In ancient times, the concept of a subterranean land inside the earth appeared in mythology, folklore and legends. The idea of subterranean realms seemed arguable, and became intertwined with the concept of "places" of origin or afterlife, such as the Greek underworld, the NordicSvartálfaheimr, the Christian Hell, and the Jewish Sheol (with details describing inner Earth in Kabalistic literature, such as the Zohar and Hesed L'Avraham). The idea of a subterranean realm is also mentioned in Tibetan Buddhism belief.[1][2] According to one story, there is an ancient city called Shamballa which is located inside the earth.[2]

Chapel, bell tower and penitential beds on Station Island. The bell tower stands on a mound that is the site of the original cave which according to various myths is an entrance which leads inside the earth to a place of purgatory. The cave has been closed since October 25, 1632.

In Celtic mythology there is a legend of a cave called "Cruachan," also known as "Ireland's gate to Hell," a mythical and ancient cave from which according to legend strange creatures would emerge in ancient times and be seen on the surface of the earth.[6] There are also stories of medievalknights and saints who went on pilgrimages to a cave located in Station Island, County Donegal in Ireland, where they made journeys inside the earth into a place of purgatory.[7] There is an Irish myth which says tunnels in County Down, Northern Ireland lead to the land of the subterranean Tuatha de Danaan, a group of people who are believed to have introduced Druidism to Ireland, and then went back underground.[8]

An ancient legend of the Angami Naga tribes of India claim that their ancestors emerged in ancient times from a subterranean land inside the earth.[9] There are legends from the Taíno people that their ancestors emerged in ancient times from two caves in a mountain underground.[10]

It is the belief of the natives of the Malinowski's Trobriand Islands that their ancestors had come from a subterranean land through a cavern hole called "Obukula".[11] There is an ancient legend held in Mexican folklore that a cave in a mountain five miles south of Ojinaga, Mexico is possessed by devilish creatures who came from inside the earth.[12]

There was an ancient myth held in the middle ages that some mountains located between Eisenach and Gotha in Germany hold a portal to the inner earth. There is an old Russian legend that says the Samoyeds, an ancient Siberiantribe, traveled to an underground cavern city to live inside the earth.[13]

According to South American mythology the belief of the Brazilian Indians, who live alongside the Parima River in Brazil, claim that their forefathers emerged in ancient times from an underground land, and that many of their ancestors still remained inside the earth. There are also legends that say the ancestors of the Inca Empire came from underground caves which are located east of Cuzco, Peru.[19]

Edmond Halley in 1692[20] put forth the idea of Earth consisting of a hollow shell about 800 km (500 mi) thick, two inner concentric shells and an innermost core, about the diameters of the planets Venus, Mars, and Mercury. Atmospheres separate these shells, and each shell has its own magnetic poles. The spheres rotate at different speeds. Halley proposed this scheme in order to explain anomalous compass readings. He envisaged the atmosphere inside as luminous (and possibly inhabited) and speculated that escaping gas caused the Aurora Borealis.[21]

De Camp and Ley have claimed (in their Lands Beyond) that Leonhard Euler also proposed a hollow-Earth idea, getting rid of multiple shells and postulating an interior sun 1,000 km (620 mi) across to provide light to advanced inner-Earth civilization but they provide no references; indeed, Euler did not propose a hollow-Earth, but there is a slightly related thought experiment.[22]

De Camp and Ley also claim that Sir John Leslie expanded on Euler's idea, suggesting two central suns named Pluto and Proserpine (this was unrelated to the dwarf planet Pluto, which was discovered and named some time later). Leslie did propose a hollow Earth in his 1829 Elements of Natural Philosophy (pp. 449–453), but does not mention interior suns.

Le Clerc Milfort in 1781 led a journey with hundreds of Creek Indians to a series of caverns near the Red River above the junction of the Mississippi river. According to Milfort the original Creek Indian ancestors are believed to have emerged out to the surface of the earth in ancient times from the caverns. Milfort also claimed the caverns they saw "could easily contain 15,000 – 20,000 families."[23][24]

In 1818, John Cleves Symmes, Jr. suggested that the Earth consisted of a hollow shell about 1,300 km (810 mi) thick, with openings about 2,300 km (1,400 mi) across at both poles with 4 inner shells each open at the poles. Symmes became the most famous of the early Hollow Earth proponents. He proposed making an expedition to the North Pole hole, thanks to efforts of one of his followers, James McBride. United States president John Quincy Adams indicated he would approve of this but he left office before this could occur. The new President of the United States, Andrew Jackson, halted the attempt.

Jeremiah Reynolds also delivered lectures on the "Hollow Earth" and argued for an expedition. Reynolds went on an expedition to Antarctica himself but missed joining the Great U.S. Exploring Expedition of 1838–1842, even though that venture was a result of his agitation.

Though Symmes himself never wrote a book about his ideas, several authors published works discussing his ideas. McBride wrote Symmes' Theory of Concentric Spheres in 1826. It appears that Reynolds has an article that appeared as a separate booklet in 1827: Remarks of Symmes' Theory Which Appeared in the American Quarterly Review. In 1868, a professor W.F. Lyons published The Hollow Globe which put forth a Symmes-like Hollow Earth hypothesis, but failed to mention Symmes himself. Symmes's son Americus then published The Symmes' Theory of Concentric Spheres in 1878 to set the record straight.

NEQUA or The Problem of the Ages, first serialized in a newspaper printed in Topeka, Kansas in 1900 and considered an early feminist utopian novel, mentions John Cleves Symmes' theory as an explanation for the hollow Earth they sail into.

An early twentieth-century proponent of hollow Earth, William Reed, wrote Phantom of the Poles in 1906. He supported the idea of a hollow Earth, but without interior shells or inner sun.

The spiritualist writer Walburga, Lady Paget in her book Colloquies with an unseen friend (1907) was an early writer to mention the hollow earth theory. She claimed that cities exist beneath a desert, which is where the people of Atlantis moved. She said an entrance to the subterranean kingdom will be discovered in the 21st century.[25]

Marshall Gardner wrote A Journey to the Earth's Interior in 1913 and published an expanded edition in 1920. He placed an interior sun in the Earth and built a working model of the hollow Earth which he patented (U.S. Patent 1,096,102). Gardner made no mention of Reed, but did criticize Symmes for his ideas. About the same time Vladimir Obruchev wrote a novel Plutonia, in which the hollow Earth possessed an inner sun and was inhabited by prehistoric species. The interior was connected with the surface by an opening in the Arctic.

George Papashvily in his Anything Can Happen (1940) claimed the discovery in the Caucasus mountains of a cavern containing human skeletons "with heads as big as bushel baskets" and an ancient tunnel leading to the centre of the earth. One man entered the tunnel and never returned.[28]

A book allegedly by a "Dr. Raymond Bernard" which appeared in 1964, The Hollow Earth, exemplifies the idea of UFOs coming from inside the earth. The book rehashes Reed and Gardner's ideas and ignores Symmes. Bernard also adds his own ideas: the Ring Nebula proves the existence of hollow worlds, as well as speculation on the fate of Atlantis and the origin of flying saucers. Bernard argued that the inhabitants of Atlantis took refuge in the Earth's interior before the city was destroyed in great calamity.[32] It was Atlanteans who piloted the flying machines known in ancient India as vimanas and in the modern world as flying saucers.[32] After the US bombings of Hiroshima and Nagasaki, Bernard claimed, the Atlanteans became concerned that radioactive air might flow into the world's interior, and so some emerged in their flying saucers in an act of self-defense.[32] An article by Martin Gardner revealed that Dr. Walter Siegmeister used the pseudonym `Bernard', but not until the publishing of Walter Kafton-Minkel's Subterranean Worlds: 100,000 years of dragons, dwarves, the dead, lost races & UFOs from inside the Earth, in 1989, did the full story of Bernard/Siegmeister become well known.[33]

The pages of the science fiction pulp magazineAmazing Stories promoted one such idea from 1945 to 1949 as "the Shaver Mystery". The magazine's editor, Ray Palmer, ran a series of stories by Richard Sharpe Shaver supposedly claimed as factual, though presented in the context of fiction. Shaver claimed that a superior pre-historic race had built a honeycomb of caves in the Earth, and that their degenerate descendants, known as "Dero", live there still, using the fantastic machines abandoned by the ancient races to torment those of us living on the surface. As one characteristic of this torment, Shaver described "voices" that purportedly came from no explainable source. Thousands of readers wrote to affirm that they, too, had heard the fiendish voices from inside the Earth. The writer David Hatcher Childress authored Lost Continents and the Hollow Earth (1998) in which he reprinted the stories of Palmer and defended the hollow earth idea based on alleged tunnel systems beneath South America and central Asia.[34]

Fantastic stories (supposedly believed as factual within fringe circles) have also circulated that Adolf Hitler and some of his followers escaped to hollow lands within the Earth after World War II via an entrance in Antarctica.[citation needed] (See also Hitler's supposed adherence to concave hollow-Earth ideas, below.)

An example of a concave hollow Earth. Humans live on the interior, with the universe in the center.

Instead of saying that humans live on the outside surface of a hollow planet — sometimes called a "convex" hollow-Earth hypothesis — some have claimed humans live on the inside surface of a hollow spherical world, so that our universe itself lies in that world's interior. This has been called the "concave" hollow-Earth hypothesis.

Cyrus Teed, a doctor from upstate New York, proposed such a concave hollow Earth in 1869, calling his scheme "Cellular Cosmogony".[40] Teed founded a group called the Koreshan Unity based on this notion, which he called Koreshanity. The main colony survives as a preserved Florida state historic site, at Estero, Florida, but all of Teed's followers have now died. Teed's followers claimed to have experimentally verified the concavity of the Earth's curvature, through surveys of the Florida coastline making use of "rectilineator" equipment.

Several twentieth-century German writers, including Peter Bender, Johannes Lang, Karl Neupert, and Fritz Braun, published works advocating the hollow Earth hypothesis, or Hohlweltlehre. It has even been reported, although apparently without historical documentation, that Adolf Hitler was influenced by concave hollow-Earth ideas and sent an expedition in an unsuccessful attempt to spy on the British fleet by pointing infrared cameras up at the sky.[41][42]

In one chapter of his book On the Wild Side (1992), Martin Gardner discusses the hollow Earth model articulated by Abdelkader. According to Gardner, this hypothesis posits that light rays travel in circular paths, and slow as they approach the center of the spherical star-filled cavern. No energy can reach the center of the cavern, which corresponds to no point a finite distance away from Earth in the widely accepted scientific cosmology. A drill, Gardner says, would lengthen as it traveled away from the cavern and eventually pass through the "point at infinity" corresponding to the center of the Earth in the widely accepted scientific cosmology. Supposedly no experiment can distinguish between the two cosmologies.

Purportedly verifiable hypotheses of a "concave hollow Earth" need to be distinguished from a thought experiment which defines a coordinate transformation such that the interior of the Earth becomes "exterior" and the exterior becomes "interior". (For example, in spherical coordinates, let radius r go to R²/r where R is the Earth's radius.) The transformation entails corresponding changes to the forms of physical laws. This is not a hypothesis but an illustration of the fact that any description of the physical world can be equivalently expressed in more than one way.[46]

The picture of the structure of the earth that has been arrived at through the study of seismic waves[47] is quite different from the hollow earth theory. The time it takes seismic waves to travel through and around the Earth directly contradicts a hollow sphere. The evidence indicates that the Earth is filled with solid rock (mantle and crust), liquid nickel-iron alloy (outer core), and solid nickel-iron (inner core).[48]

Another set of scientific arguments against a hollow Earth or any hollow planet comes from gravity. Massive objects tend to clump together gravitationally, creating non-hollow spherical objects such as stars and planets. The solid sphere is the best way in which to minimize the gravitational potential energy of a physical object; having hollowness is unfavorable in the energetic sense. In addition, ordinary matter is not strong enough to support a hollow shape of planetary size against the force of gravity; a planet-sized hollow shell with the known, observed thickness of the Earth's crust would not be able to achieve hydrostatic equilibrium with its own mass and would collapse.

Someone on the inside of a hollow Earth would not experience a significant outward pull and could not easily stand on the inner surface; rather, the theory of gravity implies that a person on the inside would be nearly weightless. This was first shown by Newton, whose shell theorem mathematically predicts a gravitational force (from the shell) of zero everywhere inside a spherically symmetric hollow shell of matter, regardless of the shell's thickness. A tiny gravitational force would arise from the fact that the Earth does not have a perfectly symmetrical spherical shape, as well as forces from other bodies such as the Moon. The centrifugal force from the Earth's rotation would pull a person (on the inner surface) outwards if the person was traveling at the same velocity as the Earth's interior and was in contact with the ground on the interior, but even the maximum centrifugal force at the equator is only 1/300 of ordinary Earth gravity.

Based upon the size of the Earth and the force of gravity on its surface, the average density of the planet Earth is 5.515 g/cm3, yet typical densities of surface rocks are only half that (about 2.75 g/cm3). If any significant portion of the Earth was hollow, the average density would be much lower than that of surface rocks. The only way for Earth to have the force of gravity that it does is for much more dense material to make up a large part of the interior. Nickel-iron alloy under the conditions expected in a non-hollow Earth would have densities ranging from about 10 to 13 g/cm3, which brings the average density of Earth to its observed value.

The deepest hole drilled to date is the Z-44 Chayvo oil well which is 12,376 m (40,604 ft)[49] deep, part of the Exxon NeftegasSakhalin-I project; thus, visual knowledge of the Earth's structure extends that far.

The idea of a hollow Earth is a common element of fiction, appearing as early as Ludvig Holberg's 1741 novel Nicolai Klimii iter subterraneum (Niels Klim's Underground Travels), in which Nicolai Klim falls through a cave while spelunking and spends several years living on a smaller globe both within and the inside of the outer shell.

The idea was used by Edgar Rice Burroughs, the creator of Tarzan, in the seven-novel "Pellucidar" series, beginning with At the Earth's Core (1914). Using a mechanical drill, his heroes discover a prehistoric world, called Pellucidar, 500 miles below the surface, that is lit by an inner sun.[50] The 1915 novel Plutonia by Vladimir Obruchev uses the concept of the hollow Earth to take the reader through various geological epochs.

In recent decades, the idea has become a staple of the science fiction and adventure genres, appearing in print; in film; on television programs such as Sanctuary, where Hollow Earth formed the core of the story arcs of the third and fourth seasons; in comics; in role-playing games such as the Hollow World Campaign Set for Dungeons & Dragons; in video games like Gears of War, where humans live on an Earth-like planet with a mostly hollow interior; and in many animated works.

^Halley, Edmond, An Account of the cause of the Change of the Variation of the Magnetic Needle; with an Hypothesis of the Structure of the Internal Parts of the Earth, Philosophical Transactions of Royal Society of London, No. 195, 1692, pp 563–578

^Halley, Edmond, An Account of the Late Surprizing Appearance of the Lights Seen in the Air, on the Sixth of March Last; With an Attempt to Explain the Principal Phaenomena thereof;, Philosophical Transactions of Royal Society of London, No. 347 (1716), pp 406–428

^Alien races and Fantastic Civilizations., Serge Hutin, Berkeley Medallion Books, 1975, pp. 109–132 – In the Bowels of the Earth: Refers to the mysterious catacombs beneath Paris, and other underground mysteries which lead inside the earth.

^Yenne, William (2003). "Adolf Hitler and the Concave Earth Cult". Secret Weapons of World War II: The Techno-Military Breakthroughs That Changed History. New York: Berkley Books. pp. 271–272. ISBN0425189929.